1. Field of the Invention
This invention relates generally to a method for removing bitumen from a carbonaceous solid containing extractable bitumen and more particularly concerns a method for removing bitumen from a carbonaceous solid containing extractable bitumen by extraction with a preselected solvent of predetermined solvency power. In another aspect, this invention relates to a method for removing fines from a carbonaceous liquid and more particularly concerns a method for removing fines from a carbonaceous liquid containing extractable bitumen by extraction with a preselected solvent of predetermined solvency power.
2. Description of the Prior Art
Various methods have been proposed in the past for the recovery of bitumen from a carbonaceous solid containing extractable bitumen, such as tar sand, diatomaceous earth or any other solid containing a heavy oil. One such method utilizes the technique of solvent extraction. A serious problem, however, in using a solvent extraction process to remove bitumen from such a carbonaceous solid is that fines, primarily particles less than 50 microns in diameter, are carried over in the solvent-dissolved bitumen extract. The presence of the fines and the failure to remove the fines result in a high-ash bitumen product as well as problems with plugging of equipment used in the separation process, for example, especially filtration equipment. Similar problems arise when other carbonaceous liquids besides bitumen, such as coal liquid or shale oil, are used. Removal of the fines during recovery of the bitumen from a carbonaceous solid or from a previously recovered carbonaceous liquid, is therefore important in providing a desirable low-ash liquid product and minimizing fouling and plugging of equipment used in the process. It would be highly desirable to develop an extraction method for recovering bitumen from the aforesaid carbonaceous solids and for removing fines from the aforesaid carbonaceous liquids which would permit control of the solvency power of the extraction solvent so as to maximize the amount of bitumen or other carbonaceous liquid recovered and to minimize the fines content therein.
In this regard, it is well known in the art that solubility parameters can be used to predict the solvency power of solvents and that the solubility parameter of a solvent system can be changed by changing the composition of the solvent system. Mitchell and Speight, Fuel, 52, 149 (1973), explored the relationship between the solubility of asphaltenes in hydrocarbon solvents and the solubility parameters of such solvents. These authors point out that there is a tendency for petroleum asphaltenes to associate in dilute solutions and that the solvent power of petroleum hydrocarbons is generally not high enough to prevent association among the asphaltenes in crude oils to aggregate into micelles. Mitchell and Speight disclose that a prime requirement for a solvent to dissolve an oil, or bitumen, in toto is the ability to penetrate the asphaltene micelle and to have a high enough solvent power to dissolve asphaltenes.
The energy that must be supplied to overcome the association forces of the micelle is supplied by the solution energy of the solvent system. Mitchell and Speight state that the Hildebrand solubility parameter is a measure of the energy which may be used to overcome the forces of the micelle. Mitchell and Speight also disclose that asphaltene precipitation can be correlated with physical properties as well as with the chemical structure of the solvent and that their results show that the physical characteristics of two different solvent types are additive on a mole-fraction basis and suggest that, when bitumen is blended with a solvent, the degree of asphaltene precipitation in the solvent might be controlled by the properties of the resulting solvent-bitumen blend.
Moreover, Mitchell and Speight disclose that a further significant fact to emerge from their investigations is the distribution of mineral matter within the asphaltene precipitate. They find that, in a two-stage process involving precipitation of part of the asphaltenes in the first stage and precipitation of the remainder of the asphaltenes in a second stage, the majority of the mineral matter originally present in the bitumen appears in the first asphaltene fraction. Mitchell and Speight state that these findings suggest that mineral matter may be removed in conjunction with a fraction relatively enriched in hetero-atoms (nitrogen, oxygen and sulphur).
Similarly, G. Fritschy and E. Papirer, Fuel, 57, 701 (1978) disclose that minerals, such as silica, alumina and kaolin, with surface hydroxyl groups, adsorb dissolved petroleum asphaltenes.